---
menu: Active Proposals
name: Customizable Select Element (Explainer)
layout: ../../layouts/ComponentLayout.astro
---

import SelectAnatomy from '../../components/select-anatomy'
import Image from '../../components/image.astro'


## Table of Contents

- [Background](#background)
- [Opting in to the new behavior](#opting-in-to-the-new-behavior)
- [HTML parser changes](#html-parser-changes)
- [Use cases](#use-cases)
  - [Customizing basic styles](#customizing-basic-styles)
  - [Rich content in `<option>`s](#rich-content-in-options)
  - [Replacing the button](#replacing-the-button)
  - [Rendering the `<option>` differently in the button](#rendering-the-option-differently-in-the-button)
  - [Putting custom content in the listbox](#putting-custom-content-in-the-listbox)
  - [Animations](#animations)
  - [Split buttons](#split-buttons)
- [Button behavior](#button-behavior)
- [The selectedoption element](#the-selectedoption-element)
- [Testing out the customizable select element](#testing-out-the-customizable-select-element)
- [Anatomy of the customizable select element](#anatomy-of-the-customizable-select-element)
- [Styling](#styling)
  - [`:open` and `:closed` pseudo-selectors](#open-and-closed-pseudo-selectors)
- [Additional examples](#additional-examples)
  - [Extending the markup](#extending-the-markup)
  - [Demo page](#demo-page)
- [Keyboard Behavior](#keyboard-behavior)
- [`multiple` and `size` attributes](#multiple-and-size-attributes)
- [Design decisions](#design-decisions)

## Background

The `<select>` element does not provide enough customization for web developers, which leads them to implement their own. These implementations can lead to reduced performance, reliability, and accessibility compared to the native form control elements. More on that is in [Custom Control UI](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/ControlUICustomization/explainer.md#introduction).

The [2020 MDN browser compatibility report had feedback](https://mdn.dev/archives/insights/reports/mdn-browser-compatibility-report-2020.html#findings-form://mdn.dev/archives/insights/reports/mdn-browser-compatibility-report-2020.html#findings-forms) that form controls are neither interoperable nor customizable. Customizable `<select>` will be fully interoperable and customizable.

## Opting in to the new behavior

The `<select>` element will continue to behave as it currently does unless it has the `appearance:base-select` CSS property. Previously proposed opt-ins included a new tag name, an additional HTML attribute, and the presence of a child `<button>` or `<datalist>`.

Here is a basic `<select>` element today:
```html
<select>
  <option>one</option>
  <option>two</option>
</select>
```

And here the same `<select>` with opt ins to the new stylability behavior which this explainer proposes:
```html
<select style="appearance:base-select">
  <option>one</option>
  <option>two</option>
</select>
```

## HTML parser changes

The current behavior in the HTML parser for `<select>` is to remove all tags which aren't `<option>` or `<optgroup>`. We will change the HTML parser to allow `<datalist>` and `<button>` as a child of `<select>`, and allow any other content within that `<datalist>` or `<button>`. The `<button>` allows authors to replace the in-page button which opens the popup, and the `<datalist>` allows authors to replace the popup element containing the options.

Here is a basic example which uses the parser changes:
```html
<select style="appearance:base-select">
  <button>
    <selectedoption></selectedoption>
  </button>
  <datalist>
    <option>one</option>
    <option>two</option>
  </datalist>
</select>
```

## Use cases

### Customizing basic styles

This example changes the fonts and colors of various parts of the button and listbox parts of the select element.

<div className="code-image-container">
<div className="code-container">
```html
<select>
  <datalist>
    <option>one</option>
    <option>two</option>
  </datalist>
</select>
<style>
select datalist,
select button {
  font-family: monospace;
  font-size: 12px;
}
</style>
```
</div>
<div className="image-container">
  <Image
    src="/images/selectlist-usecase-basic.png"
    alt="The rendering of a customizable select element with monospaced font and red colors"
  />
</div>
</div>

### Rich content in `<option>`s

This example adds "rich content" inside option elements in the form of country flags. This is a common pattern on the web which isn't possible in the existing `<select>` element because `<select>`'s `<option>`s only render text content.

<div className="code-image-container">
<div>
```html
<select>
  <datalist>
    <option>
      <img src="usa.jpg" alt="flag of USA">
      USA
    </option>
    <option>
      <img src="germany.jpg" alt="flag of Germany">
      Germany
    </option>
    <option>
      <img src="spain.jpg" alt="flag of Spain">
      Spain
    </option>
  </datalist>
</select>
```
</div>
<div>
  <Image
    src="/images/selectlist-usecase-rich.png"
    alt="The rendering of a customizable select element with country flags next to the options"
  />
</div>
</div>

### Replacing the button

This example replaces the button which opens the listbox with an author provided `<button>` element.

<div className="code-image-container">
<div>
```html
<select>
  <button>
    selected option: <selectedoption></selectedoption>
  </button>
  <option>one</option>
  <option>two</option>
</select>
```
</div>
<div>
  <Image
    src="/images/selectlist-usecase-button.png"
    alt="The rendering of a customizable select element with an author-provided button"
  />
</div>
</div>

### Rendering the `<option>` differently in the button

This example uses the customizable `<select>` element with custom CSS to target it which renders the option differently in the listbox vs in the button.

<div className="code-image-container">
<div>
```html
<select>
  <button>
    <selectedoption></selectedoption>
  </button>
  <datalist>
    <option>
      ❤️ <span class=description>heart</span>
    </option>
    <option>
      🙂 <span class=description>smile</span>
    </option>
  </datalist>
</select>
<style>
selectedoption .description {
  display: none;
}
</style>
```
</div>
<div>
  <Image
    src="/images/selectlist-usecase-selectedoption.png"
    alt="The rendering of a customizable select element with a custom selectedoption element"
  />
</div>
</div>

### Putting custom content in the listbox

This example has a `<datalist>` which enables it to put arbitrary content into the listbox rather than just `<option>`s.

<div className="code-image-container">
<div>
```html
<select>
  <datalist>
    <div class=container>
      <div>
        <optgroup label="1-2">
          <option>one</option>
          <option>two</option>
        </optgroup>
      </div>
      <div>
        <optgroup label="3-4">
          <option>three</option>
          <option>four</option>
        </optgroup>
      </div>
      <div>
        <optgroup label="5-6">
          <option>five</option>
          <option>six</option>
        </optgroup>
      </div>
      <div>
        <optgroup label="7-8">
          <option>seven</option>
          <option>eight</option>
        </optgroup>
      </div>
    </div>
  </datalist>
</select>
<style>
.container {
  display: grid;
  grid-template-rows: auto auto;
  grid-template-columns: 50px 50px;
  grid-column-gap: 10px;
  grid-row-gap: 10px;
}
.container > div {
  background-color: lightgray;
}
</style>
```
</div>
<div>
  <Image
    src="/images/selectlist-usecase-listbox.png"
    alt="The rendering of a customizable select element with a custom listbox element"
  />
</div>
</div>

Here is another example with custom content in the listbox: [codepen](https://codepen.io/una/pen/PoXbgVW)

### Animations

<div className="code-image-container">
<div>
  This example uses view transitions to animate the opening and closing of the listbox.
  [Link to codepen](https://codepen.io/argyleink/pen/wvYrZEV/ab71f1b613db0487f64ff8c6919b0173)
</div>
<div>
  <Image
    src="/images/selectlist-animation.gif"
    alt="A customizable select element with a listbox that animates open and closed"
  />
</div>
</div>

### Split buttons

<div className="code-image-container">
<div>
  This example uses the [selectedoptionelement](#the-selectedoptionelement-attribute) attribute in order to have separate buttons which open the select's popover and another one which has the selected option content and submits a form. The image is taken from a richer example here: [link to codepen](https://codepen.io/una/pen/KKbNYbo).
```html
<button><selectedoption id=myselectedoption></selectedoption></button>
<select selectedoptionelement=myselectedoption>
  <button>⬇️</button>
  <datalist>
    <option>Create a merge commit</option>
    <option>Squash and merge</option>
    <option>Rebase and merge</option>
  </datalist>
</select>
```
</div>
<div>
  <Image
    src="/images/select-split-button.png"
    alt="A customizable select element with a split button"
  />
</div>
</div>

## Button behavior

The first child `<button>` of a `<select>` will be slotted into the `<select>`'s UA ShadowRoot and will open the popup list of options.

## The `<selectedoption>` element

The `<selectedoption>` element enables developers to declaratively clone the content of the selected `<option>` element to the button and to style it distinctly from the `<option>`. This use case is shown in the [rendering the option differently in the button](#rendering-the-option-differently-in-the-button) example.

When the `<selectedoption>` element is placed inside of a `<select>`'s `<button>`, then the browser will replace its DOM contents with the result of calling `cloneNode()` on the currently selected `<option>` element. Whenever the user selects a different `<option>` or when there is a DOM mutation inside of the selected `<option>`, the browser will update the contents of `<selectedoption>` by internally calling `cloneNode()` again.

Since the contents of the `<selectedoption>` element are maintained by the browser, authors should not put any HTML/DOM content inside of `<selectedoption>`.

## The `selectedoptionelement` attribute

`<select>` will support the `selectedoptionelement` attribute, which is an IDref which points to a single `<selectedoption>` element to update. This allows the `<selectedoption>` to exist outside of `<select>` in order to support the split buttons use case.

## Testing out the customizable select element

Customizable `<select>` is currently implemented behind a flag in [Chromium](https://chromestatus.com/feature/5737365999976448). To use it, enable the **Experimental Web Platform features** flag in about:flags.

If you encouter bugs or limitations with the design of the control, please send your feedback by [creating issues on the open-ui GitHub repository](https://github.com/openui/open-ui/issues/new?title=[select]%20&labels=select). Here is a list of [open select bugs in open-ui](https://github.com/openui/open-ui/issues?q=is%3Aissue+is%3Aopen+label%3Aselect).

## Anatomy of the customizable select element

Because the various parts of the select element can be styled, it's important to understand the anatomy of this UI control.

<SelectAnatomy />

- `<select>` - The root element that contains the button and listbox.
- button (slot) - The portion of the element which is rendered in the position of the button which opens the listbox. It should contain a button to open the listbox.
- `<button>` - The button which opens the listbox when clicked.
- `<selectedoption>` - The element which contains the text of the currently selected option. Every time that the user selects an option, the browser will replace the children of this element with the result of calling `cloneNode()` on the newly selected `<option>`.
- `<datalist>` - The wrapper that contains the `<option>`(s) and `<optgroup>`(s).
- `<optgroup>` - Optional element which groups `<option>`(s) together with a label.
- `<legend>` - Used to provide a label to an `<optgroup>` element. It must be the first child of the `<optgroup>`.
- `<option>` - Can have one or more and represents the potential values that can be chosen by the user.

## Styling 

Customizable `<select>` provides a variety of tools to help with styling, including pseudo-selectors for different states and pseudo-elements for different parts.

### `:open` and `:closed` pseudo-selectors

The select element supports the `:open` and `:closed` pseudo selectors as specified by CSS here: https://drafts.csswg.org/selectors/#open-state

When the select's listbox is showing, the select element will match `:open`, and when it is not showing, it will match `:closed`. Here is an example which makes the button red when the listbox is closed and green when it is open:
```html
<select>
  <button id=custombutton>
    <selectedoption></selectedoption>
  </button>
  <option>one</option>
  <option>two</option>
</select>
<style>
select:open #custombutton {
  background-color: green;
}
select:closed #custombutton {
  background-color: red;
}
</style>
```

## Additional examples

Here is an example which has a custom button with custom CSS:

```html
<style>
  .my-custom-select button {
    display: flex;
    align-content: center;
  }
  .my-custom-select .open {
    padding: 5px;
    border: none;
    background: #f06;
    border-radius: 5px 0 0 5px;
    color: white;
    font-weight: bold;
  }
  .my-custom-select .label {
    padding: 5px;
    border: 1px solid #f06;
    border-radius: 0 5px 5px 0;
  }
</style>
<select class="my-custom-select">
  <button>
    <span class="open">Open</span>
    <span class="label">Choose an option</span>
  </button>
  <option>Option 1</option>
  <option>Option 2</option>
  <option>Option 3</option>
</select>
```

The above code snippet results in the following style:

<Image
  src="/images/selectlist-replacing-button-part.png"
  alt="The rendering of a select element with a custom button"
/>

Here is an example which has a custom listbox with custom CSS:

```html
<style>
  .my-custom-select datalist {
    width: 300px;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(100px, 1fr));
    gap: 10px;
    padding: 10px;
    box-shadow: none;
    margin: 10px 0;
    border: 1px solid;
    background: #f7f7f7;
  }
</style>
<select class="my-custom-select">
  <datalist>
    <option>Option 1</option>
    <option>Option 2</option>
    <option>Option 3</option>
    <option>Option 4</option>
    <option>Option 5</option>
  </datalist>
</select>
```

The above code snippet results in the following style:

<Image
  src="/images/selectlist-replacing-listbox-part.png"
  alt="The rendering of a select element with a custom listbox"
/>

### Extending the markup

Not only can you replace the default parts with your own, you can also extend the control's markup by adding new elements. This can be useful to augment the listbox or button with extra information, or to add new functionality.

Consider the following example:

```html
<style>
  .my-custom-select button {
    display: flex;
    align-items: center;
    gap: 1rem;
    border: none;
    background-color: transparent;
  }
  .my-custom-select .icon {
    border: none;
    margin: 0;
    padding: 0;
    width: 2rem;
    height: 2rem;
    border-radius: 50%;
    display: grid;
    place-content: center;
    background-color: buttonface;
  }
  .my-custom-select .icon::before {
    content: '\25BC';
  }
  .my-custom-select datalist {
    padding: 0;
  }
  .my-custom-select .section {
    padding: 1rem 0 0;
    background: radial-gradient(ellipse 60% 50px at center top, #000a 0%, transparent 130%);
  }
  .my-custom-select h3 {
    margin: 0 0 1rem 0;
    text-align: center;
    color: white;
  }
  .my-custom-select option {
    text-align: center;
    padding: 0.5rem;
  }
</style>
<label for=plant>Choose a plant</label>
<select id=plant class="my-custom-select">
  <button>
    <selectedoption></selectedoption>
    <span class=icon></span>
  </button>
  <datalist>
    <div class="section">
      <h3>Flowers</h3>
      <option>Rose</option>
      <option>Lily</option>
      <option>Orchid</option>
      <option>Tulip</option>
    </div>
    <div class="section">
      <h3>Trees</h3>
      <option>Weeping willow</option>
      <option>Dragon tree</option>
      <option>Giant sequoia</option>
    </div>
  </datalist>
</select>
```

Using custom markup to wrap the list of options, the above example creates sections with their own styles and content as seen below:

<Image
  src="/images/selectlist-using-custom-markup.png"
  alt="The rendering of a select element with custom HTML content"
/>

### Demo page

You can find multiple examples of customizable select on our [demo page](https://microsoftedge.github.io/Demos/selectmenu/).

## Keyboard Behavior

When a custom `<datalist>` element is provided, the `<select>` element has unified keyboard behavior. The new behavior is inspried by both the existing `<select>` element and the [Aria Authoring Practices Guide Combobox Pattern](https://www.w3.org/WAI/ARIA/apg/patterns/combobox/). This behavior was decided in [issue 386](https://github.com/openui/open-ui/issues/386) and [issue 433](https://github.com/openui/open-ui/issues/433).

When the listbox is closed and the button is focused:
* Spacebar opens the listbox.
* Enter submits the form associated with the select if one exists. Otherwise, enter does nothing.
* The up, down, left, and right arrow keys all open the listbox without changing the selected value.

When the listbox is open:
* Escape closes the listbox without changing the selected value.
* Spacebar is used for typeahead.
* Enter changes the selected value to the currently focused option and closes the listbox.
* The up arrow key moves focus backwards in the list of options and changes the selected value to the newly selected option.
* The down arrow key moves focus forwards in the list of options and changes the selected value to the newly selected option.
* The left and right arrow keys do nothing.

## `multiple` and `size` attributes

The HTML parser will not allow `<button>` or `<datalist>` children when the `multiple` or `size` attributes are present on `<select>`. This will ensure that the old rendering behavior of `multiple` and `size` is used. If the author adds a `<datalist>` or `<button>` via script to a `<select multiple>` or `<select size>`, the old rendering will still be used. In the future, we would like to upgrade `<select multiple>` to become customizable, at which time we will change the HTML parser again to allow `<button>` and `<datalist>` children. This also allows feature detection to know whether `<select multiple>` improvements are ready to be used. More detail in [this issue](https://github.com/openui/open-ui/issues/977#issuecomment-1910874971).

## Checkmark

A checkmark is included next to the option elements by default in order to visually indicate which option is currently selected. This improves accessibility/usability because selection doesn't follow focus. You can opt out of rendering the checkmark with the following css:
```css
option::before {
  display: none;
}
```

## Design decisions

- [Why reuse `<select>` instead of creating a new element](https://github.com/openui/open-ui/issues/970)
- [Why not use the `slot` or `behavior` attributes](https://github.com/openui/open-ui/issues/702)
- [Why not support the `size` attribute](https://github.com/openui/open-ui/issues/153)
- [Why support `:checked` instead of `:selected`](https://github.com/openui/open-ui/issues/827)
- [Why allow interactive content in `<listbox>` but with strong warnings](https://github.com/openui/open-ui/issues/540)
- [Why not make selection follow focus](https://github.com/openui/open-ui/issues/742)
- [Why not allow select or datalist in size/multiple select](https://github.com/openui/open-ui/issues/977#issuecomment-1910874971)
- [Why add the `<selectedoption>` element](https://github.com/openui/open-ui/issues/571)
- [`<selectedoption>` name bikeshedding](https://github.com/openui/open-ui/issues/808)
- Why opt-in with CSS
  - https://github.com/whatwg/html/issues/9799#issuecomment-1789202391
  - https://github.com/whatwg/html/issues/9799#issuecomment-1926411811
- [Why include a checkmark next to options by default](https://github.com/openui/open-ui/issues/863)
- [Why `<button type=popover>` (overturned by `selectedoptionelement`)](https://github.com/openui/open-ui/issues/939#issuecomment-1910837275)
- [Why `selectedoptionelement`](https://github.com/openui/open-ui/issues/1063#issuecomment-2195407344)
